1. Field of the Invention
The present invention relates to a functional device such as a so-called organic electroluminescent device, which converts electric energy into light for emission and can be used for a variety of light sources in a variety of display devices, electrophotography, illumination, and the like, to useful materials therefor and to a method for manufacturing the organic electroluminescent device.
2. Description of the Related Art
Today, research and development of a variety of display devices is aggressively being carried out. For example, organic electroluminescent devices (hereinafter sometimes referred to as “organic EL devices”), which have an organic thin film formed by vapor deposition of organic compounds (for example, see Applied Physics Letters, 51, pp. 913-915 (1987)) are known. Such organic electroluminescent devices can produce light with high luminance under low voltage and therefore receive attention as promising display devices.
However, a method of manufacturing organic EL devices by vacuum deposition of organic compounds has a problem with respect to productivity. In particular, it is difficult to form organic EL device having a large area by a doping process, because, in the doping process, different materials must be vapor-deposited at the same time with a concentration of each material precisely controlled. In addition, amorphous films of low-molecular materials have poor heat resistance and tend to cause defects such as crystallized portions.
In order to solve such problems, it has been proposed that the low-molecular organic layers should be replaced with high-molecular organic polymer layers, and a variety of organic electroluminescent devices which include a charge transport group-incorporated polymer chain have also been reported. Examples of such a polymer include a π conjugated system, such as poly(paraphenylene vinylene) (for example, see Advanced Materials, p. 4 (1992)). A method of producing devices by coating such a material is suitable in terms of simplification of the process, processibility and production of devices having a large area, and is promising because the resultant device has good heat resistance.
The polymers with the π conjugated system are excellent materials, because they are coatable and have good heat resistance. Since a luminescent portion is in a conjugated main chain of the polymer, however, it is difficult to control a concentration of the luminescent material, and it is also difficult to delicately control color tone and luminescent intensity. In addition, thermal motion of the polymer is caused as the temperature increases with the emission, and the conjugated structure can be disturbed, so that color tone may tend to vary.
Another example of a light emitting device similarly produced through the coating step is a device in which a low-molecular weight fluorescent compound is dispersed in poly (N-vinylcarbazole) (for example, see Japanese Patent Application Laid-Open (JP-A) No. 4-212286). This device is a function separation type-device in which the poly(N-vinylcarbazole) has a charge transport function and the low-molecular weight fluorescent compound has a light emitting function. In this type of device, the type of the fluorescent compound can freely be altered, and therefore, color tone and luminescent intensity can be relatively easily adjusted. Since the poly(N-vinylcarbazole) has a relatively large carbazole group at a side chain, however, it forms a brittle film or it is difficult to form a film with it. In addition, a driving voltage is high, and the durability of such a device is problematic, because brightness tends to decrease when the device is driven after a long period of non-use or is driven continuously.
In view of such problems, polycarbonate having a repeating unit of a diarylene-vinylene-arylene skeleton has been proposed for an organic electroluminescent device produced through the coating step (for example, see JP-A No. 5-247458). It is disclosed that this material can be formed into a thin film by coating and can provide a blue light emitting organic EL device. However, the material has a problem in that film-forming properties are insufficient, a uniform surface is hard to form, and durability is low because of dark spot generation.
In order to solve the above problems, polyethers, polythioethers, and polyesters each having a repeating unit of a diarylene-vinylene-arylene skeleton are disclosed as a material for use in a light emitting device (for example, see JP-A No. 2001-40347). In the device made of such a material, however, the polymer tends to crystallize, because the ether or ester group is directly bonded to the arylene group in the skeleton having the light emitting function. Such a material cannot be sufficient in terms of solubility and film-forming properties.
Several examples have also been proposed in which a polymer whose skeleton has an electron transport function as well as a light emitting function is used to form a film. For example, an oxadiazole skeleton having a high electron transport performance and EL devices in which the oxadiazole is incorporated into a main chain of a polyether or into a side chain of a polyvinyl type polymer are known (for example, see JP-A Nos. 9-188756 and 9-255725).
In general, the oxadiazole skeleton-incorporated polymer has low solubility in an organic solvent and tends to have poor film-forming properties, and these drawbacks are improved by copolymerizing the oxadiazole with a highly soluble structure. In such a case, however, it is difficult to obtain sufficient electron transport performance. Other reported examples include vapor deposition and polymerization of a reactive monomer having the oxadiazole structure in the process of forming a thin film (for example, see JP-A No. 8-88084). However, such a process has a problem with respect to productivity because of the difficulty in controlling a reaction rate of the reactive group, and it is hard to achieve sufficient durable properties due to dark spot generation and the like.
Thus, there has been a demand for a polyester resin that has excellent coatability, adhesion to substrates and durability, and that is a useful material for forming a light emitting device that efficiently produces light with high brightness under low driving voltage. There has also been a demand for a functional device, an organic electroluminescent device and a method of manufacturing the organic electroluminescent device, which each uses such a material.